The clock governing such mammalian circadian rhythms as the sleep-wake or activity-rest cycles is located in the hypothalamic suprachiasmatic nucleus (SCN). Recently, several studies have demonstrated that areas within a single SCN oscillate out of phase with respect to one another. In order to properly understand the spatial issues relating to phasic activity at the cellular level in the SCN, it is necessary to have a careful description of the SCN itself. Despite the very large volume of literature on the topic, there has been no complete, three dimensional evaluation of the SCN intrinsic anatomy in any species. The vast majority of studies have used adjacent tissue sections to compare relative spatial locations of cell types within the nucleus; many fewer experiments have employed double label methods to determine whether individual cells contain two distinct neuromodulators. The proposed work will use immunohistofluorescence techniques to study combinations of three labels in order to determine their specific locations in the SCN. Importantly, all the work will be performed with animals pretreated with colchicine.Preliminary data show that all SCN neurons are darkly immunoreactive for GABA. This neuromodulator will serve to define the nucleus and anchor the locations of cells immunoreactive (IR) to other neuromodulators. There are neuron groups in the hamster SCN immunoreactive to GAGA, substance P, calbindin, calretinin, vasoactive intestinal polypeptide, vasopressin, cholecystokinin, and gastrin releasing peptide. The distribution of GABA cells encompasses the entire nucleus, vasopressin and cholecystokinin neurons occupy a dorsomedial location; substance P, gastrin releasing peptide, calbindin and calretinin cells are found in a central subnucleus; and vasoactive intestinal polypeptide neurons are found in a ventrally located area. There appears to be fairly extensive overlap among certain of the cell distributions. The exact locations of the cell types and their locations of overlap and co-localization of neuromodulator will be determined, and function will be assessed by evaluating co-localization of light-induced protein markers. In addition, the locations of the various cell types will be determined with respect to the terminal fields of the three main SCN-afferent projections, the NPY-IR geniculohypothalamic tract from the intergeniculate leaflet, the serotonin-lR projection from the median raphe nucleus and the retinohypothalamic projection identified by cholera toxin B fragment-IR following an eye injection. Presynaptic terminal identity will be confirmed by the presence of synaptophysin-IR. Work on the hamster will be extended to the mouse and we will determine which cell types exhibit light-induced gene expression in response to nocturnal light exposure. The results will provide the anatomical framework necessary for interpretation of future physiological and molecular experiments.